1. Field of the Invention
The present invention relates to a wire electric discharge machining apparatus in which a wire electrode is vertically supported under tension between upper and lower wire guides. In particular, the present invention relates to a method of measuring a taper angle of the wire electrode.
2. Description of the Related Art
A wire electric discharge machining apparatus is widely known which cuts a plate-like workpiece in a manner analogous to a band saw by supplying a series of current pulses to a moving wire electrode. Taper cut is known as cutting using a wire electrode which is inclined by an instructed taper angle by moving one of the wire guides relative to the other in a horizontal XY plane.
Japanese Laid-open utility model application 3-103119 discloses a method of measuring a taper angle of wire electrode with a measuring tool. As illustrated in FIG. 1A, a measuring tool 1 is conventionally used to measure a taper angle of wire electrode. The measuring tool 1 is made of conductive material and includes a rectangular body 10 and two parallel arms 12 and 16. The arms 12 and 16 extend longitudinally from one side of the rectangular body 10 and have respective sharp edges 13 and 17. The sharp edges 13 and 17 extend longitudinally and parallel and are vertically spaced by a given height H. The bottom of the measuring tool 1 includes the lower edge 17. The upper and lower sharp edges 13 and 17 are laterally spaced and face each other.
A conventional method of measuring a taper angle will now be described with reference to FIGS. 1A and 1B.
The measuring tool 1 is positioned on an XY table T so that the edges 13 and 17 extend in the direction of Y-axis. The XY table T is moveable in the directions of orthogonal X and Y axes. A wire electrode can be moved within an XY plane relative to the measuring tool 1 by movement of the XY table T.
A wire electrode is aligned vertically to an XY plane between upper and lower wire guides GU and GL. The upper wire guide GU is moveable in the directions of orthogonal U and V axes. The U-axis is parallel to the X-axis and the V-axis is parallel to the Y-axis. The vertical wire electrode is positioned at a reference position R between the sharp edges 13 and 17 by movement of the XY table T. Coordinates (x, y) of the reference position R are stored.
The vertical wire electrode EV is moved in a negative X-axis direction. At step S1, when electrical contact between the wire electrode EV and the upper sharp edge 13 is detected, the XY table T is stopped and a first X-axis distance X1 is stored. The wire electrode EV is returned to the reference position R.
The wire electrode EV is moved in a positive X-axis direction. At step S2, when electrical contact between the wire electrode EV and the lower sharp edge 17 is detected, the XY table T is stopped and a second X-axis distance X2 is stored. The wire electrode EV is returned to the reference position R.
The wire electrode is inclined at an instructed taper angle θ by moving the upper wire guide GU in a positive U-axis direction. A U-axis distance U of the upper wire guide GU is stored.
The inclined wire electrode EI is moved in a negative X-axis direction. At step S3, when electrical contact between the wire electrode EI and the upper sharp edge 13 is detected, the XY table T is stopped and a third X-axis distance X3 is stored. The wire electrode EV is returned to the reference position R.
The wire electrode EI is moved in a positive X-axis direction. At step S4, when electrical contact between the wire electrode EI and the lower sharp edge 17 is detected, the XY table T is stopped and a fourth X-axis distance X4 is stored.
A taper angle θ is expressed by an equation (1):θ=tan−1 γ/H  (1)γ is expressed by an equation (2):γ=α−β  (2)α and β are expressed by equations (3) and (4), respectively:α=|(X3+R/cos θ)−(X1+R)|  (3)β=|(X2+R)−(X4+R/cos θ)|  (4)
R is a radius of wire electrode. Based on the equations (2), (3) and (4), γ is expressed by an equation (5):γ=(|X3|+|X4|)−(|X1|+∥X2∥)+2R(1/cos θ−1)  (5)The value γ may be inaccurate as the equation (5) includes taper angle θ of which actual value has not been obtained.